The present invention relates to a method of lapping both surfaces of a wafer to eliminate defects such as irregular shape and work damages, and an apparatus therefor. The present invention is particularly suitable for processing a large wafer.
A single crystal ingot manufactured by CZ or FZ method is cut into blocks having a predetermined shape, ground at the periphery and then set in a slicer. In the slicer, the crystal block is sliced into wafers having a predetermined thickness by a high-speed rotating blade.
A slicing blade is representatively an inner diameter saw prepared by cutting a thin stainless steel sheet to a doughnut shape and depositing a Ni plating layer in which diamond abrasives are embedded on an inner edge of the shaped stainless steel sheet.
A wafer obtained by slicing a crystal ingot is likely to deviate in thickness and flatness due to various conditions such as a tension applied to a blade, an adhering state of diamond abrasives onto an inner edge of the blade and dimensional accuracy of a rotation axis of a slicer. If slicing conditions are not appropriate, a work damage layer which extends from the surface is deeply developed into the inner part of the sliced wafer.
These unfavorable deviations resulting from slicing are eliminated by lapping a sliced wafer.
In a conventional lapping method, a plurality of wafers 1 are set in carriers 2 and located on a lower lapping plate 3, in the manner such that the wafers 1 are uniformly distributed on the lower lapping plate 3, as shown in FIGS. 1(a) and 1(b). An upper lapping plate 4 is brought down into contact with the wafers 1, abrasives are fed into a gap between the lower lapping plate 3 and the upper lapping plate 4, and the wafers 1 are rotated and revolved. During rotation and revolution, the wafers 1 are ground with the abrasives. A commonly used slurry is prepared by suspending Al.sub.2 O.sub.3 or SiC grains as abrasives having a particle size of approximately 10 .mu.m in a proper amount of water.
Various methods have been proposed so far, to facilitate setting of wafers in such a lapping machine. For instance, wafers are sucked to an upper lapping plate and then located on a lower lapping plate at proper positions, as disclosed in Japanese Patent Publication 56-189.
Recently, semiconductor wafers are becoming larger and larger in size, in order to enhance the productivity of electronic devices. In this regard, there is a strong demand for enlargement of a lapping machine to a scale suitable for lapping large wafers or for provision of another type of lapping machine which can lap such large wafers.
If large wafers are lapped by such a conventional lapping machine as shown in FIGS. 1(a) and 1(b), it is necessary to use lapping plates having a radius larger than a diameter of wafers. Consequently, the lapping machine as a whole would be very large in size, various problems on weight, size, operatability etc. would result and automatic operation or control would be extremely difficult.
In addition, a difference in the linear velocity between outer and inner parts of lapping plates increases in proportion to a diameter of lapping plates. The velocity difference causes uneven abrasion of the lapping plates, so that the surface of lapped wafers lack uniformity.
A lapping machine suitable for lapping a large wafer is proposed in Japanese Patent Publication 52-12956. The proposed lapping machine has a pair of disc-shaped or ring-shaped lapping plates which are pressed onto both surfaces of a wafer. While the lapping plates pressed onto the wafer are rotated, a slurry in which abrasive grains are suspended is supplied downwards to the wafer between the lapping plates at the top of the lapping plates.
However, the abrasive slurry is not sufficiently carried to gaps between the lapping plates and the wafer, since the abrasive slurry is fed from the outside of the lapping plates. The abrasive slurry is not effectively used for lapping the wafer, and only a small amount of the abrasive slurry is carried to the inside of the lapping plates.
For instance, in the case using ring-shaped lapping plates having cavities at their central parts, a downward flow of the abrasive slurry can not be restricted at a lower part of the lapping plate opposite to a slurry supply position. Consequently, a lack of abrasives results at the lower parts of the lapping plates. Although the lack of abrasives could be avoided by increasing the rotation speed of the wafer, the wafer such as a semiconductor wafer does not have mechanical strength sufficiently resistant to such a high speed rotation.
The present invention is accomplished to overcome these problems.
The first object of the present invention is to provide a lapping machine suitable for easily lapping or polishing both surfaces of a wafer, even one which has a large diameter, to a smooth state with high lapping efficiency, and without enlarging the scale of the lapping machine.
The second object of the present invention is to increase the ratio of an abrasive slurry which is effectively consumed for lapping a wafer.
The third object of the present invention is to promote discharge of silicon dusts, which are separated from both surfaces of a wafer being lapped, through ring-shaped lapping plates having contact planes narrow in width.